Stop detector for rotary device

ABSTRACT

A stop detector for detecting a stop of a rotary device comprises a reed switch mounted on a fixed frame of the rotary device along the axis of its rotor and a pair of travelling magnets arranged so as to move perpendicularly to the axis of the rotor when centrifugal force is exerted. The travelling magnets move along a pair of non-magnetic guides fixed on the rotor and the non-magnetic guides are equipped with a pair of repulsing members at their far ends to give repulsive force to the travelling magnets when the rotor is halted thereby actuating the reed switch.

United States Patent n91 Hasegawa et al.

Masamichi Nekotsuka; Kazushige Muramatsu; Yukiyoshi Yamada, all

of Sagamihara, Japan [73] Assignee: Mitsubishi Denki Kabushiki Kaisha.

Tokyo, Japan [22] Filed: Oct. 17, 1974 [211 App]. No.1 515,480

[30] Foreign Application Priority Data Nov, 22 1973 Japan H 48-135596Nov, 22, 1973 Japan ..48-135597 Jan. 17, 1974 Japan 49-8564 [52] US. Cl335/207; 340/271 [51] Int. Cl. HOIH 36/00 [58] Field of Search 335/207288; 200/80: 340/264. 271; 310/68 E [56] References Cited UNITED STATESPATENTS 3,147.350 9/1964 Lindley, Jr 335/207 Oct. 28, 1975 3.67.3,5276/1972 Wolf 335/207 Primary E.mminerHarold Broome Attorney. Agent, orFirmOblon, Fisher, Spivak McClelland & Maier [57] ABSTRACT A stopdetector for detecting a stop of a rotary device comprises a reed switchmounted on a fixed frame of the rotary device along the axis of itsrotor and a pair of travelling magnets arranged so as to moveperpendicularly to the axis of the rotor when centrifugal force isexerted. The travelling magnets move along a pair of non-magnetic guidesfixed on the rotor and the non-magnetic guides are equipped with a pairof repulsing members at their fair ends to give repulsive force to thetravelling magnets when the rotor is halted thereby actuating the reedswitch 8 Claims, 11 Drawing Figures US. Patent Oct. 28, 1975 Sheet 2 of33,916,361

M1; iilmml NS SN DISTANCE OF MOVEMENT OF TRAVELLING MAGNET, mm N u a 01as l i l l 0 500 I000 I500 2000 RATE OF REVOLUTION, RPM

U.S. Patent Oct. 28, 1975 Sheet 3 of3 3,916,361

w J |lJ SN. N S. rlL F'L .I m 5 SN N S M w 4 4 E Em 9565: a NEE FIG.11

DEPTH INSERTED, mm

STOP DETECTOR FOR ROTARY DEVICE BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates to a novel stop detector fordetecting a disconnection of a rotary device such as a cooling fan.

2. Description of the Prior Art Cooling fans have been widely used invarious fields of electrics and electronics for the purpose ofpreventing overheating, shortness of life, and change ofcharacteristics. The operation period of a cooling fan is usually quitelong and sometimes a cooling fan is required to operate continuously forseveral years. If a cooling fan fails, it is necessary that it berepaired or replaced as soon as possible. Accordingly, it has beenproposed that a cooling fan be equipped with means for indicating amalfunction thereof.

The following types of detecting elements have been known.

a. devices for detecting wind volume (for example a combination heaterand thermal sensitive element) b. devices for detecting wind pressure c.devices for detecting the rate of revolution (for example a combinationsearch coil and semiconductor logical circuit).

However, these devices have disadvantages and there is no satisfactorydevice having high reliability and low cost. For example, in thedevices, it is difficult to set the operation temperature since it isaffected by atmospheric temperature. In the a and b devices, the effectof cooling fans of adjacent apparatus causes complications. The 0devices have unusually complicated structures and are quite expensive.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide a novel stop detector for overcoming the abovenoted disadvantages.

The foregoing and other objects are attained in accordance with oneaspect of the present invention through the provision of a stop detectorfor a rotary device which comprises a rotary member having a rotorcomprising a non-magnetic central block having a center hole whose axiscoincides with the axis of the rotor, the central block being fixed tothe rotor of the rotary member, two non-magnetic guides bonded to thecentral block to extend symmetrically on an axis perpendicular to theaxis of the center hole, a pair of travelling magnets freely fitted oneon each non-magnetic guide so as to magnetically repulse each other, apair of repulsing members mounted at each end of the nonmagnetic guides,a stationary member comprising a reed switch, a supporter mounted to theframe of the rotary member, the reed switch being fixed to the supporterwith one end of said reed switch mounted freely into the center hole ofthe central block.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plane view of oneembodiment of a stop detector for a rotary member according to theinvention;

FIG. 2 is a sectional view taken along the line AA in FIG. 1;

FIG. 3 is a sectional view of a magnetic circuit used for a basicmeasurement;

III

FIG. 4 is a characteristic graph showing a balanced position by arepulsive force and a centrifugal force;

FIG. 5 is a plane view of another embodiment of the invention;

FIG. 6 is a sectional view taken along the line AA in FIG. 1;

FIG. 7 is a sectional view of a magnetic circuit used for a basicmeasurement;

FIG. 8 is a characteristic graph showing a balanced position by arepulsive force and a centrifugal force;

FIG. 9 is a sectional view of another embodiment of the invention;

FIG. 10 is a schematic view of a reed switch used in the invention;

FIG. 11 is a performance characteristics curves of the stop detector fora rotary member acccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of theinvention will be illustrated referring to FIGS. 1 and 2 wherein thereference numeral l designates a reed switch which is held by asupporter 7 mounted on a frame 8 for the rotary member and the end ofthe switch is freely inserted into a center hole 9 of the central block2. The reed switch 1 is fixed to the supporter 7. The central block 2 isfixed to the rotor 6 and is made of non-magnetic material. Thereferences 3, 3 represent non-magnetic cylinders fixed to the centralblock 2; 4, 4' represent cylindrical travelling magnets having an outerdiameter which is slightly smaller than the inner diameter of thenon-magnetic cylinder; 5, 5' designate repulsing members such asrepulsing magnets fixed at ends of the non-magnetic cyl inders and whichhave the same size as the cylindrical travelling magnets 4, 4. Thecylindrical travelling magnets 4, 4' and the repulsing magnets S, 5 aremagnetized in the axial direction. The directions of the magnetic polesare selected to be repulsive for 5 to 4; 4 to 4' and 4' to 5' as shownin FIG. 2.

When the rotor 6 formed by the rotating parts 2-5 is rotated, thecentrifugal force acts upon the travelling magnets 4, 4' whereby thetravelling magnets 4, 4' move against the repulsive force of therepulsing magnets 5, 5. A desirable stop detector can be achieved bydesignating the magnetic circuit so that the travelling magnets 4, 4'turn on the switch I when the rotor stops and turn off the switch 1 whenthe rotor reaches its desired speed.

It should be emphasized that it is unnecessary to turn the switch 1 onor off depending upon the rotation of the cooling fan. In the case of arate of revolution of 2,200 r.p.m., the life of the reed switch will beonly one month because the life of a commercial reed switch is onlyabout IO0,000,000 ON-OFF times in the conventional structure which turnson by detecting the approach of the magnets fixed to the rotary member.

However, the life of the stop detector of the invention is quite long.In the specification, the rotary member, the stationary member and thereed switch are defined as follows. The rotary member comprises thecentral block 2; the travelling magnets 4 4' and the repulsing members5, 5 (magnets or springs) and the non-magnetic guides 3, 3'. Thestationary member comprises the reed switch I and the supporter 7.

The reed switch comprises two magnetic flexible electrodes which arenormally separated but which contact each other when a magnetic fieldhaving a certain strength is applied. The electrode can have a free endwhich can be inserted into the center hole if desirable and the reedswitch itself can be inserted into the central hole. In the centralhole, the electrode can be magnetized by the travelling magnets as theyapproach the electrode causing the electrodes to contact each other toactuate the reed switch.

An experiment will now be described. FIG. 3 is a sectional view of amagnetic circuit for a basic measurement showing the operation of theinvention. A glass tube having an inner diameter of 5.1 mm and a lengthof 42 mm was used as the non-magnetic cylinders 3, 3'. Four isotropicalbariumferrite magnets having an outer diameter of mm and a length of 5.9mm which were magnetized as desired were inserted in the glass tube andthe two magnets at the end were fixed to the glass tubes 3, 3' to beused as the repulsing magnets 5, 5'. In the normal state, the travellingmagnets 4, 4', the two inner magnets, repulsed each other setting up adistance therebetween. The same distances were established for 5 to 4; 4to 4'and 4' to 5'.

The glass tubes 3, 3' were bonded perpendicularly and symmetrically inthe direction of the axis of rotation as shown in FIG. 2 to the surfaceof the rotor 6 of the cooling fan which had a rated revolution of 2,200r.p.m. The cooling fan was rotated by applying a rated voltage and theinside of the glass tubes 3, 3' were observed using a stroboscopicmethod. It was found that the two travelling magnets 4, 4' were shiftedto the far ends. When the rate of revolution was decreased by decreasingthe applied voltage by variable voltage regulator (transformer), it wasobserved by decreasing synchronous frequency of the stroboscope that thetravelling magnets 4, 4' approached each other with the decreasing rateof revolution as shown in FIG. 4.

Next, the turn-on and turn-off states of the reed switch (glass tubehaving an outer diameter of 3 mm and a length of 20 mm; about 30 AT)were observed by using the travelling magnets 4, 4' having the same sizeas abovementioned. The maximum distance between the positions of themagnets from the turn-off state to the turn-on state was 12 mm; and theminimum distance was 16 mm.

Finally, the stop detector shown in FIG. 2 was composed by using a reedswitch, a glass tube having an inner diameter of 5.1 mm, the centralblock 2 and the supporter 7 made of bakelite (phenolformaldehyde resin).The relation between the turn-on and -off state of the reed switch andthe rate of revolution was measured. in the experiment, the total lengthof the rotating member was 42 mm so as to coincide with the firstexperiment. The response of the reed switch was observed by changing therate of revolution. The change from the turn-on state to the turn-offstate occurred at about 920 r.p.m. and the change from the turn-offstate to the turn-on state occurred at about 650 r.p.m.

The data are highly dependent upon the inserted depth of the reed switch1 into the center hole 9 but are not substantially dependent upon theposition of the stop detector (vertical and horizontal direction). As isclear from the experiment, it is possible to form a stop detector whichworks at a desirable rate of revolution of the rotor by combining thereed switch and the magnets in accordance with the invention.

In the experiment, in order to apply a force against the centrifugalforce to the travelling magnets 4, 4', the

repulsing magnets 5, 5' were used as repulsive members. However, it ispossible to replace the repulsing magnets by coil springs using thefollowing calculation. The centrifugal force f is given by the equation:

wherein m represents the mass [g] of the travelling magnets 4, 4'; rrepresents the distance of the two magnets from the central axis whenthey are balanced [cm]; 1 represents the rate of revolution expressed inangular frequency and 1 represents the length of the travelling magnets4, 4'.

The restoring force f, of a coil spring is given by the equation.

f, 31 k (r r,,) [dyne] wherein r, represents the natural length of thecoil spring'[cm]; k represents a constant of the coil spring [dyne/cm].

The balanced position can be given by the equations 1 and 2;

When the magnets are used as travelling magnets 4, 4' and the reedswitch 1 is set so as to turn-off at 1000 r.p.m. wherein r 8 6 14 [cm],the centrifugal force 1, is estimated as about 8000 dyne. If r, 6 mm,the constant k is about 9.050 [dyne/cm]. This means that a coil springhaving a restoring force of about 1 g per l cm of contraction must beused. It is possible to provide such a coil spring by using a copperalloy wire having a diameter of 0.15 mm.

In the above-mentioned experiment or calculation, typical size magnetswere utilized. However, it is possible to alter the size to useanisotropic ferrite magnets or Alnico magnets instead of isotropicferrite magnets. The coil spring can be replaced by a conical spring ora spiral spring. This is a matter of design. Only the inner diameter ofthe non-magnetic cylinder is critical. Accordingly the outer surface isnot limited to a circular sectional view but can be in the shape of arectangle or a hexagon.

Another embodiment of the invention will be illustrated referring toFIGS. 5 and 6 wherein the reference numeral 1 designates a reed switchwhich is held by a supporter 7 mounted on a frame 8 for the rotarymember. The end of the switch is freely inserted into a center hole 9 ofthe central block 2 and the switch 1 is fixed to the supporter 7. Thecentral block 2 is fixed to the rotor 6 and is made of non-magneticsliding shafts fixed to the central block 2; 12, 12, 12 representcylindrical travelling magnets having an inner diameter which isslightly larger than the outer diameter of the non-magnetic cylinder;l3, l3 designate repulsing members such as repulsing magnets fixed atthe ends of the sliding shafts which are the same size as thecylindrical travelling magnets 12, 12'. The travelling magnets 12, 12and the repulsing magnets 13, 13' are magnetized in the axial directionof the non-magnetic sliding shafts 11, 11 and the directions of themagnetic poles are selected to be repulsive for 13 to 12; 12 to 12' and12' to 13' as shown in FIG. 6.

When the rotating parts 12-13 are rotated, centrifugal force is appliedto the travelling magnets 12, 12' causing the travelling magnets 12, 12'to move against the repulsive force of the repulsing magnets 13, 13'. Adesirable stop detector can be obtained by designing the magneticcircuit so that the travelling magnets 12, 12 tum-on the switch 1 whenrotation ceases and tumoff the switch 1 when the rated speed isachieved.

An experiment will now be described. FIG. 7 is a sectional view of amagnetic circuit for a basic measurement showing the operation of theinvention. A brass rod having a diameter of 2 mm and a length of 42. mmwas used as the non-magnetic sliding shafts 11, 11'. Four isotropicalbarium-ferrite magnets having an outer diameter of S, 6 mm, an innerdiameter of 2.5 mm and a length of 5.9 mm which were magnetized asdesired were fitted to the rod. The two magnets at the ends were fixedto the far ends of the brass rods 11, 11' so as to function as repulsingmagnets 13, 13'. In the normal state, the travelling magnets 12, 12, thetwo inner magnets, repulsed each other so as to establish a distancetherebetween. Substantially the same distances were established for 13to 12; 12 to 12' and 12' to 13. The magnetic circuits were bondedperpendicularly and symmetrically in the direction of the axis ofrotation as shown in FIG. 6 to the surface of the rotor 6 of the coolingfan which rotates at a rated revolution of 2,200 r.p.m. The cooling fanwas rotated by applying a rated voltage and the position of thetravelling magnets 12, 12' was observed using a stroboscopic methodwhereby it was found that two travelling magnets 12, 12' were shifted tothe far ends.

When the rate of revolution was decreased by decreasing the appliedvoltage through the use of a variable voltage regulator, it was observedby decreasing the synchronous frequency of the stroboscope that thetravelling magnets 4, 4' approached each other as the rate of revolutiondecreased as shown in FIG. 8.

Next, the turn-on and turn-off states of the reed switch (glass tubhaving an outer diameter of 3 mm and a length of 20 mm; about 30 AT)were observed by using travelling magnets 12, 12' having the same sizeas above-mentioned. The maximum distance between the positions of themagnets from the tum-off state to the tum-on state was 1 1.5 mm; and theminimum distance was l5.5 mm.

The stop detector shown in FIG. 6 was composed by using a reed switch, abrass rod having a diameter of 2 mm, the central block 2 and thesupporter 7 made of a bakelite (phenol-formaldehyde resin). The relationbetween the tum-on and turn-off state of the reed switch and the rate ofrevolution was measured. In the experiment, the total length of therotor was 42 mm so as to coincide with the first experiment. Theresponse of the reed switch was measured by changing the rate ofrevolution. The change from the turn-on state to the tum-off stateoccurred at about 900 r.p.m. and the change from the turn-off state tothe turn-on state was at about 620 r.p.m. The data were highly dependentupon the inserted depth of the reed switch 1 into the center hole 9, butthe data were not substantially dependent upon the position of the stopdetector. (vertical and horizontal direction).

An embodiment of the stop detector wherein the reed switch and themagnets are combined has been proposed. The stop detector responds whenthe rotary member reaches a certain rate of revolution which is lowerthan a desired rate of revolution. However, in the stop detector, thereed switch which is freely inserted into the center hole of the centralblock is comprised of a brittle glass tube. The accuracy of theclearance for a free fit is therefore quite important. If the reedswitch is assembled with too large a clearance the glass tube may bebroken.

Another embodiment of the invention which is suitable from theabove-mentioned standpoint will now be described with reference to FIGS.9 to 11. In FIG. 9, the reference numeral 2 designates a central blockhaving a center hole 9 which is made of non-magnetic material; 3, 3designate non-magnetic guides such as brass tubes which are mountedsymmetrically to the central block 2 and vertically to the center hole9; 4, 4 designate travelling magnets which may be, for example, ofcylindrical shape and which are respectively freely fitted to thenon-magnetic guides 3, 3", 5, 5' designate repulsing members such asrepulsing magnets which are respectively fixed at each end of thenonmagnetic guides 3, 3'.

The rotary member of the stop detector comprises parts 1-5' and is fixedto the rotor 6. Its center coincides with the rotary shaft 10a of therotor 6 and the central axis of the center hole 9. The reference numeral7 is a supporter mounted to the frame 8 for the rotor 6 and 1 designatesa reed switch. The lower end of the switch is inserted into the centerhole 9 and the upper end of the switch is fixed to the supporter 7. Thesupporter 7 and the frame 8 form the fixed part of the stop detector.The travelling magnets 4, 4' and the repulsing magnets 5, 5' aremagnetized in the axial direction and the directions of the magneticpoles are selected to be repulsive for 5 to 4; 4 to 4' and 4' to 5. Inthe rest state, the travelling magnets 4, 4' are disposed at a balancedposition with respect to the central block.

When the rotor 6 is rotated, centrifugal force is applied to thetravelling magnets 4, 4' causing the travelling magnets 4, 4 to moveagainst the repulsive force generated by the magnets 5, 5. The reedswitch 1 detects the movement of the travelling magnets 4, 4' and turnson when the rotor 6 ceases rotation and turns off when the rated speedis attained. Thus, the magnetic circuit is dependent upon the balance ofthe centrifugal force and the repulsive force.

The minimum distance and the maximum distance between the travellingmagnets 4, 4' are set depending upon the response (ampere turns) of thereed switch 1. The response depends upon the selection of the reedswitch 1 and also the depth of insertion into the center hole 9.Accordingly, data is secured by experimentation.

In the embodiment shown in FIG. 10 the reed switch 1 has an asymmetricalshape. The reed switch 1 has a long electrode 14 and a short electrode15 sealed in a glass tube. In the asymmetrical switch 1, it is possibleto tum-on the switch by causing the travelling magnets 4, 4' to approachthe magnetic wire 16 of the short electrode as shown in FIG. 10.Accordin y, In the embodiment shown in FIG. 10, the part of 5 feedswitch which is inserted into the center hole 9 is IiBi the glen: tubebut is the magnetic wire 16 of the shaft elec t'rbdi 15.

From the view point of the magnetic circuit, if there is equivalencewhether the travelling magnets 4, 4' are at the positions of the fullline or the dotted line in FIG. 10, it is to be understood that the reedswitch 1 can be utilized as stated above. The advantages of the use ofthe reed switch 1 are as follows:

i. It is unnecessary to accurately adjust the clearance of the centerhole 9 and the reed switch 1.

2. The tolerances for the parts are rather large so that the cost of theparts and the assembly thereof can be decreased.

3. There is no difficulty even though the free fit is altered by longoperation. Accordingly, the reliability of the device is improved.

4. The travelling magnets 4, 4' can approach closer to the reed switch 1so that the magnetic gap is small. Accordingly, the size of thetravelling magnets 4, 4 can be made small and the total size can becompacted.

5. The wiring from the magnetic leading wire 16 is a relatively simplematter.

One experiment with respect to this embodiment will now be described. Abrass tube having an outer diameter of 5.5 mm, an inner diameter of 3,mm and a length of 35 mm was prepared and a center hole 1a having adiameter of 3 mm was formed in the tube. An opening having a diameter of4.2 mm was drilled to a depth of mm from the ends of the brass tube inthe center of the step shape. In the experiment, the central block 2 andthe non-magnetic guides 3, 3' were formed in one piece.

Four isotropic barium ferrite magnets having a diameter of 4 mm and alength of 5 mm were magnetized as desired and were inserted into thebrass tube to form the rotary part of the stop detector. The stopdetector was prepared by using a commercial reed switch 1 (glass tubehaving a diameter of 2 mm and a length of 9 mm; response AT of -35;otT-set type), a supporter 7 made of bakelite (phenolformaldehyde resin)and a cooling fan having a rated revolution of 2,200 rpm.

The vertical position of the reed switch 1 was made moveable and ameasurement was made for the relation between the depth of insertion ofthe magnetic wire 16 into the center hole 9 and the the rotatingrevolutions per minute for the stop detector.

FIG. 11 shows the results of measurements. The pharmetery designates thetotal length of the magnetic wire 16 and the short electrode 15. Asshown in FIG. 11, is not necessary that x, y, be extremely accuratebecause the detector can satisfactorily respond at speeds of 600-900r.p.m. As it is clear from the above statements, the stop detector ofthe invention has a remarkably high reliability and a rather low cost.

Thus, the industrial value of the invention is remarkably high.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by letters patent ofthe United States is:

l. A stop detector for a rotary device which comprises:

a rotary member having a rotor comprising:

a non-magnetic central block having a center hole whose axis coincideswith the axis of the rotor; the central block being fixed to the rotorof the rotary member;

two non-magnetic guides bonded to the central block to extendsymmetrically on an axis perpendicular to the axis of the center hole;

a pair of travelling magnets freely fitted one on each non-magneticguide so as to magnetically repulse each other;

a pair of repulsing members mounted at each end of the non-magneticguides;

a stationary member comprising a reed switch;

a supporter mounted to the frame of the rotary member;

the reed switch being fixed to the supporter with one end of said reedswitch mounted freely into the center hole of the central block.

2. The stop detector according to claim 1 wherein the non-magneticguides comprise cylindrical tubes and the travelling magnets arecylindrical.

3. The stop detector according to claim 1 wherein the non-magneticguides comprise guide shafts and the travelling magnets each have aguide hole.

4. The stop detector according to claim 1 wherein the reed switchcomprises a first electrode and a second electrode sealed in a tube, oneend of the second electrode being disposed without the tube, said oneend of the second electrode being freely mounted in the center hole.

5. The stop detector according to claim 1 wherein the repulsing memberscomprise coil springs.

6. The stop detector according to claim 5 wherein the coil springscomprise copper based alloys.

7. The stop detector according to claim 3 wherein the non-magnetic guideshafts comprise cylindrical rods and the travelling magnets comprisecylindrical tubes.

8. The stop detector according to claim 1 wherein the non-magneticguides and the central block comprise one piece.

1. A stop detector for a rotary device which comprises: a rotary memberhaving a rotor comprising: a non-magnetic central block having a centerhole whose axis coincides with the axis of the rotor; the central blockbeing fixed to the rotor of the rotary member; two non-magnetic guidesbonded to the central block to extend symmetrically on an axisperpendicular to the axis of the center hole; a pair of travellingmagnets freely fitted one on each nonmagnetic guide so as tomagnetically repulse each other; a pair of repulsing members mounted ateach end of the nonmagnetic guides; a stationary member comprising areed switch; a supporter mounted to the frame of the rotary member; thereed switch being fixed to the supporter with one end of said reedswitch mounted freely into the center hole of the central block.
 2. Thestop detector according to claim 1 wherein the non-magnetic guidescomprise cylindrical tubes and the travelling magnets are cylindrical.3. The stop detector according to claim 1 wherein the non-magneticguides comprise guide shafts and the travelling magnets each have aguide hole.
 4. The stop detector according to claim 1 wherein the reedswitch comprises a first electrode and a second electrode sealed in atube, one end of the second electrode being disposed without the tube,said one end of the second electrode being freely mounted in the centerhole.
 5. The stop detector according to claim 1 wherein the repulsingmembers comprise coil springs.
 6. The stop detector according to claim 5wherein the coil springs comprise copper based alloys.
 7. The stopdetector according to claim 3 wherein the non-magnetic guide shaftscomprise cylindrical rods and the travelling magnets comprisecylindrical tubes.
 8. The stop detector according to claim 1 wherein thenon-magnetic guides and the central block comprise one piece.